I believe somebody have installed the Disable Comments and had also applied to the persistent mode: https://wordpress.org/plugins/disable-comments/faq/
I found the answer from the author of the plugin and it might help you:
https://wordpress.org/support/topic/how-to-re-enable-comments-after-using-persistent-mode?replies=3
(@tane63)
9 years, 7 months ago
I am unable to make the comments turn on. The posts say comments off, but I have checked every box I can find to turn them on! I have gone under the discussion settings and checked the box to allow anyone to comment. I cannot find a place under individual comments to turn them on for each post.
I have deactivated all of my plug-ins in case that was it and I have changed my theme in case that was the cause. Nothing seems to help. What am I missing!